Geological grouting is a versatile construction technique used in a variety of applications. Injection casing or piping is driven into the ground. Grout is then pumped under pressure through the above-ground end of the installed casing, out the underground end, and into the surrounding soil. The grout itself can be made from many different materials proportioned in a wide range of amounts depending on the specific grouting application. Cementitious grout, for example, is a mixture of hydraulic cement and water, with or without aggregates and with or without admixtures. Hydraulic cements react with water to form a hardened paste that maintains its strength and durability in water and also maintains its properties upon drying.
Grouting applications include slabjacking, subsealing and soil grouting. In slabjacking, pressure grouting is used to raise a depressed section of pavement or other concrete element by forcing a flowable grout under it. Subsealing is where a cement-grout mixture is pumped under pressure through a packer installed in an access hole drilled in a slab to fill voids and depressions under the slab and reduce damage caused by excessive pavement deflections. For soil grouting, soil is grouted to increase its bearing capacity, reduce or halt settlement, increase shear resistance to stabilize it against lateral movement, reduce waterflow, or increase the cohesive strength of friable ground prior to excavation. Soil grouting includes permeation grouting, where a thin grout is used to permeate the soil and fill pores and voids between soil particles; deep-soil mixing, where soil and injected grout are mixed together to make a soil-cement material in place; jet grouting, where a cement-and-water grout is injected under very high pressure to form a concrete-like column; and compaction grouting, described below.
Compaction grouting is a soil stabilization process where weak or compromised sub-soils are re-compacted. This technique involves driving injection casing into the soil in five to eight foot sections until good refusal is achieved, usually when the casing reaches bedrock or bearing strata. Pressure grouting is then performed in vertical stages throughout the length of the casing hole. The vertical stages are created by extracting a section of casing a fixed length, typically one to three feet, and then pumping a quantity of stiff, sand-and-cement grout through the casings. An operator monitors an external pressure gauge and pump stroke counter at a pump head attached to the casing end. The operator also records the pressures achieved and the quantity of grout injected at each stage. A fully extracted section of casing is removed between stages, the pump head is reattached, and the extraction and grouting sequence is repeated. The stiff grout does not permeate the soil but maintains a grouted mass, three feet or more in diameter. By displacing the soil and forming a bulblike or columnlike form, the grout significantly increases the soil density at a radial distance of one to six feet or more from the soil-grout interface.